Our broad, long-term objective is to understand on a molecular level the mechanism of action of anticancer drugs with an overall goal of improving current chemotherapy and identification of novel targets and drugs. The role that Bcl-2 proteins play in microtubule inhibitor-induced apoptosis is of special interest. Bcl-2 proteins are subject to complex regulation, and one of the most prominent post-translational modifications is the phosphorylation of Bcl-xL and Bcl-2 occurring in response to microtubule inhibitors. However, the role of these modifications and the kinase(s) responsible have remained largely obscure, despite their potential importance as a mechanistic link between microtubule dysfunction and apoptosis induction. Preliminary studies using KB-3 cells have indicated that vinblastine-induced phosphorylation of Bcl-xL and Bcl-2 are coordinated events catalyzed by the same kinase and that their dephosphorylation correlates with apoptosis initiation. Our hypothesis is that Bcl-xL/Bcl-2 phosphorylation is a key event controlling apoptosis induction by antimitotic drugs and is catalyzed by a novel kinase activated in response to microtubule dysfunction. In Specific Aim 1, the sites of phosphorylation in Bcl-xL and Bcl-2 in response to vinblastine treatment of KB-3 cells will be identified by protein purification, trypsin digestion, and mass spectrometry analysis. In Specific Aim 2, the role of Bcl-xL/Bcl-2 phosphorylation in vinblastine-induced apoptosis will be evaluated by expressing phosphorylation-defective or phosphorylation-mimic molecules and examining cellular sensitivity to vinblastine. In Specific Aim 3, mechanistic studies will be performed to determine whether phosphorylation affects Bcl-xL or Bcl-2 subcellular localization or protein/protein interactions. In Specific Aim 4, the vinblastine-activated Bcl-xL/Bcl-2 kinase will be purified and characterized, with a long-term goal of understanding its function and regulation in the cellular response to microtubule damage. This study will provide novel insight into the role of Bcl-xL/Bcl-2 phosphorylation in the mechanism of action of vinblastine and other antimitotic drugs and provide important basic information on regulatory mechanisms of Bcl-2 protein function. These findings in turn will aid in the search for superior chemotherapeutic drugs, new drug targets, and methods to overcome tumor cell resistance to these agents.